1. Field of the Invention
The present invention relates to an image checking device that compares a read image with a reference image so as to perform image checking, to a printing system that includes the image checking device, to an image checking method, and to a computer program product.
2. Description of the Related Art
Document printing is performed by using, for example, a method for directly spraying ink onto a sheet of paper on the basis of an original document or a method for transferring ink onto a sheet of paper by using, as a medium, a plate that is created on the basis of an original document. Methods using a plate include relief printing, planographic printing, intaglio printing, and screen printing. Out of these methods, planographic printing predominates in commercial printing because it offers advantages in that it can represent images that are close to the original images and create high-quality printed material.
Planographic printing is offset printing in which ink is attached to a plate with images formed thereon and the plate with the ink image is brought into contact with a blanket for transferring so that the ink image is printed on a sheet of paper via the blanket. Offset printing is a printing technology in which an image is not directly printed from the plate onto the sheet of paper but is printed via a medium, such as a blanket. A device that uses this technology is an offset printer and is used in the printing industry, or the like, as a printer that can print any number of copies of the same material once the plate is made.
Some offset printers have an image checking device that compares an image of the printout read by an image reading device with a prepared reference image so as to perform image checking. Installing this image checking device makes it possible to determine whether a print is acceptable and to maintain a constant image quality of printed material.
However, the degree of ink blur is different depending on the type of paper of printed material. For example, copy paper with superior smoothness, such as commonly used high-quality paper, has a different degree of ink blur from Japanese paper that has a fibrous surface; therefore, there is a problem in that the quality of a printed image varies depending on the type of paper. Furthermore, the level of image quality may be different depending on the content of information (a thin line image, a solid image, or the like) to be printed.
In recent years, on-demand printing, where printing is performed immediately after a command is received, has been put into practical use. Image checking is also required for on-demand printing. With on-demand printing, the details of printing may change for each copy; therefore, it is impossible to prepare a reference image. Thus, it is necessary to create a reference image from the print data; however, because the print data does not include information about the type of paper, it is difficult to create a reference image with the accuracy that is required for image checking.
In view of the above problems, a device is disclosed which can set details for checking and acceptance criteria for printed material in accordance with the type of printing medium, such as the type of paper (for example, Japanese Patent Application Laid-open No. 2005-205703). Information about the trays, the type of printing medium contained in each tray, and checking information for checking an image formed on each printing medium are stored in this device in an associated manner, and the criteria, which are set for the printing medium with the image formed thereon, are used for checking the printed material by comparing an image read from the printed material with the reference image after the image has been formed, whereby it can be determined whether the printed material is acceptable on the basis of the checking criteria, which are set for each printing medium.
In the above-described device, because the checking criteria are changed in accordance with the type of paper, it is possible to provide printed material with a constant image quality regardless of the type of paper.
However, in the above-described conventional technology, the difference that occurs between the read image and the reference image is not only due to the type of paper but also the different degrees of change in the densities of highlights and shadows depending on the type of paper; thus, if the type of paper is changed, the above device cannot ensure a constant checking accuracy for all of the density ranges from highlights to shadows. Thus, there is a need to provide an image checking device that can correspond to such changes in the density.